What I am planning to do, and what others have suggested, is to shop Goodwill or whatever thrift shop you have in the area and get a full-sized blender for your shell crushing convenience, versus a coffee bean grinder. Also - ask friends and neighbors. For some reason, people keep all manner of gadgets that they’re no longer using. ;)
I let the shells dry out thoroughly, then put them in a heavy Ziplock bag, then use a meat mallet (flat side) to crush them up. then they go into an old, covered, ice cream tub for use in the garden.
But, your idea with the gravel in a laundry detergent bottle, and shaking, sounds like a good ‘Field Expedient Method.’ (Army-Speak)
And Diana’s idea of shopping the thrift stores for a used blender is probably the best economical idea!
And here I went and passed up a medium size blender in very good condition at a yard sale a few days ago...
In fact, if it had been a bit rough and $6 (hahaha), I would have bought it, but, I was thinking about the wear too. It was also sort of an off brand, so I wasn’t sure about replacement blades. Plus I’d already spent more than I intended — darn seller had too much really useful stuff! Like, a “cast net” in good condition for $5 — if one lives near many lakes & ponds, and 2 large rivers, that could be a lifesaver in a real SHTF scenario. And a big chicken feeder for $8. And...
However... It dawned on me after I left that although it’d be a shame to beat up a nice blender, for smashing egg shells, etc., who cares about blade and canister wear if it’s basically a “dedicated” unit?
RATS! Maybe I’ll stop back by there and see if these people still have the blender. They kinda know me by now, as they run yard sales every year and I usually stop by. They even saved string for a trimmer I bought from them, last yard sale. (They’d mentioned having it but not on hand. Instead of selling it they figured I’d be by again sometime! :-) )
I could try Goodwill, too.
In the meantime, I think I’ll try the gravel in a laundry detergent bottle, and report back... ;-)